Intracranial pressure (ICP) is the pressure within the cranium and reflects the pressure experienced by brain tissue. The body has various mechanisms by which it keeps the ICP stable, particularly by controlling cerebrospinal fluid (CSF) pressure through production and absorption of CSF. ICP is measured in millimeters of mercury (mmHg) and, at rest, is normally 7-15 mmHg for a supine adult, and becomes negative (averaging −10 mmHg) in the vertical position. Changes in ICP are attributed to volume changes in one or more of the constituents contained in the cranium.
One of the most damaging aspects of brain trauma and other head injuries is an elevated ICP. An increase in ICP, most commonly due to head injury leading to intracranial hematoma or cerebral edema, can crush brain tissue, shift brain structures, contribute to hydrocephalus, cause the brain to herniate and restrict blood supply to the brain. Additionally, it can be a cause of reflex bradycardia.
Elevated ICP reduces cerebral perfusion pressure (CPP) and if uncontrolled results in vomiting, headaches, blurred vision, or loss of consciousness. Further elevation in ICP can cause permanent brain damage and eventually a fatal hemorrhage at the base of the skull. An elevated ICP in excess of about 20 mm HG, in adults, is termed pathologic intracranial hypertension (ICH) and is considered a medical/surgical emergency. Particular instances where it is desirable to monitor ICP are in traumatic brain injury (TBI) victims, stroke victims, hydrocephalus patients, patients undergoing intracranial procedures, patients with brain tumor, “shaken baby” syndrome, kidney dialysis, or artificial liver support.
It is also possible for the ICP to drop below normal levels, though increased intracranial pressure is far more common and far more serious. The symptoms for both conditions are often the same, leading many medical experts to believe that it is the change in pressure rather than the pressure itself that results in the above symptoms.
Current ICP monitoring techniques are generally grouped as either invasive or non-invasive. The invasive group is further divided into soft tissue techniques, for example lumbar puncture, and cranial invasive techniques. The latter comprises three distinct methods for monitoring ICP:                An intraventricular catheter, which is a thin, flexible tube threaded into one of the two lateral ventricles of the brain;        A subarachnoid screw or bolt placed just through the skull in the space between the arachnoid membrane and cerebral cortex; and        An epidural sensor placed into the epidural space beneath the skull.        
In a lumbar puncture or spinal tap, a clinician delicately passes a fine needle through the lower region of the back into the fluid of the spinal cord. Once the spinal spaces have been penetrated, ICP can be estimated by attaching a pressure sensor. The communication between the fluid in the spinal column and the cranium allows the physician to ascertain the pressure in the cranium responsive to CSF pressure. Though invasive, a lumbar puncture is sometimes preferred because it is a soft tissue procedure rather than a cranial procedure. Generally, a non-neuro clinician will not feel comfortable performing a cranial procedure, but will perform a lumbar puncture. This procedure does allow transient manipulation or sampling of the intracranial fluid system, but is often painful and many times results in after affects, and typically raises patient apprehension. Additionally, it is a short term procedure and is generally not considered useful for long term ICP monitoring.
The cranial invasive techniques, although medically accepted and routinely used, suffer from several drawbacks. In particular, the transducer has to be calibrated in some fashion before insertion. The placement of the system requires a highly trained individual; in almost all clinical settings, this procedure is limited to physicians, and in most cases further limited to a specialist such as a neurosurgeon. This generally limits these procedures to larger medical facilities. Furthermore, there is a relatively short-term (32-72 hours) reliability and stability of the system, due to a number of causes including: leaks; plugging of the transducer; inadvertent disturbance of the transducer; or inadvertent removal of the transducer. This concern generally limits these procedures to intense monitoring setting such as an ICU. There are also associated risks of invasive transducer placement such as brain or spinal cord damage and infection. Even though these risks are low, these concerns generally limit the group of invasive ICP monitoring techniques to a hospital setting and prevents standard use of the techniques in clinic or nursing home settings.
In the non-invasive group, the accepted, commercially available method of monitoring ICP consists of taking a CT, MRI, or other image of the head, interpreting the image and observing changes in various features. This method requires a high level of skill to read and assess the images and requires that the patient be brought to the imaging equipment. In many cases, a scan is delayed or cancelled because the patient is not stable enough to be moved. Even after the patient is stable, the various tubes and equipment connections to the patient have to be accounted for during transport to the relevant imaging equipment, and as a result additional personnel may be required, with a consequent increase in cost. In addition, the scans themselves are single measurements-“snap-shots” in time, of which at least two are required to assess subtle changes and variations. A series of scans could approximate continuous monitoring, but is not economically practical.
Other methods include the estimation of the pressure using a combination of transcranial Doppler (TCD) ultrasound equipment, which is designed to assess cerebral blood flow velocities and estimation of the optic nerve sheath diameter. Such techniques are taught for example in U.S. Patent Application Publication Ser. No. 2011/0137182 published Jun. 9, 2011 to Bellezza and Lai, the entire contents of which is incorporated herein by reference. Unfortunately, detection of optic nerve sheath diameter is difficult to perform automatically, and requires a skilled clinician to properly identify the appropriate nerve.
U.S. Pat. No. 5,919,144 issued Jul. 6, 1999 to Bridger et al., the entire contents of which is incorporated herein by reference, is addressed to a non-invasive apparatus and method for measuring ICP. An acoustic signal is transmitted through the skull of a patient and the properties of the transmitted signal after propagation through the skull are measured and correlated with ICP, particularly changes in resonant frequency response are monitored. Unfortunately, observing change in resonant frequency does not provide for a sufficiently accurate measurement of ICP. Furthermore, the technique of Bridger has not succeeded in achieving wide use after more than a decade.
U.S. patent application publication US 2008/0200832 published Aug. 21, 2008 to Stone, the entire contents of which are incorporated herein by reference, is addressed to a non-invasive ICP monitoring system and method. The system includes an auditory stimulation and recording unit which includes a stimulation controller, a memory for storing waveforms, a device for comparing waveforms with store waveforms and an alarm operable based on the comparison. The system includes at least one cranial electrode attachable to a patient, and an auditory stimulation device such as a pair of acoustic ear inserts. A patient is auditorially stimulated via the auditory stimulation device to evoke a received waveform. The received waveform is compared with an established patient baseline waveform or an established normal waveform to generate ICP information. Unfortunately, this does not provide an accurate direct measurement of ICP over a range of patients, as it is only a comparison to baseline data.
U.S. Pat. No. 6,387,051 issued May 14, 2002 to Ragauskas, the entire contents of which are incorporated herein by reference, is addressed to a to a non-invasive ICP monitoring system and method. A broadband ultrasound signal is transmitted through the skull and detected by a sensor. The received broadband signal is decomposed into narrowband components. Each component is analyzed and the ICP is determined. Unfortunately, the requirement for ultrasonic equipment adds to cost, particularly as it requires highly trained personnel for appropriate operation.
Thus, there is a long felt need for a non-invasive device operative to provide a direct measurement of ICP, providing improved accuracy and not required trained personnel.